Having sat quietly at first, does get fidgety (5)
I believe the answer is:
posed
'fidgety' is the definition.
The answer and definition are different parts of speech. However, adjectives and past participle verbs can occasionally define each other.
'having sat quietly at first does' is the wordplay.
I cannot really understand how this works, but
'quietly' could be 'p' (p for piano and quiet) and 'p' is present in the answer.
an anagram of 'does' is 'osed' which is present in the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
'get' acts as a link.
Can you help me to learn more?
(Other definitions for posed that I've seen before include "Positioned" , "Faced the camera" , "Sat (for artist)" , "Struck attitude - sat for painter" , "Modelled" .)